Acid mine drainage (AMD) has severe, long lasting impacts on water quality and stream ecology in affected watersheds. Despite remedial actions, the overall ecology of some impacted areas has not rebounded due to accumulation of iron(III) (oxy)-hydroxides on the streambed, which is thought to cut off critical geochemical gradients within the hyporheic zone. Presence of thick AMD deposits on a streambed have potential to alter flowpaths and residence time of water within bed sediments, which are strong determinants of the (bio)geochemical transformations and potential contaminant attenuation that occur in streams. This work used an interdisciplinary approach, applying hydrogeological, mineralogical, and geochemical methods to determine how hydro-geomorphic settings affect changes within AMD sediments and affect mineralogical and geochemical gradients along flow paths.  In a southeastern Ohio watershed impacted by historical coal mining, the study examined two reaches, one with a debris dam and one with step-pools. In both reaches, flow paths within sediments strongly impact (bio)geochemical gradients: downwelling causes surface water to become deoxygenated as it passes through and interacts with sediments containing freshly deposited material derived from AMD generation, resulting in accumulation and transformation of more crystalline Fe-bearing materials (dominated by goethite) to poorly crystalline material. During upwelling below the debris dam, deoxygenated water continues to alter local geochemical conditions through reductive dissolution of Fe-bearing phases and subsequent re-precipitation of poorly crystalline material. Groundwater-stream exchange in the presence of AMD-derived materials results in a more complicated picture of sediment mineralogy on the streambed than previously observed. Further work is needed to closely associate fine scale variations in hydraulic head and conductivity with water and sediment chemistry and mineralogy within the vertical profile.

The relationship between hydro-geomorphic setting and geochemical gradients along flowpaths in stream sediments impacted by acid mine drainage

255th National Meeting (2018)

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